Astroparticle Physics

158 7 Secondary Cosmic RaysFig. 7.26Average longitudinal developmentof the various components of anextensive air shower in theatmospherein which the extensive air shower develops. The atmosphereconstitutes approximately a target of 11 interaction lengthsand 27 radiation lengths. The minimum energy for a primaryparticle to be reasonably well measured at sea levelvia the particles produced in the air shower is about 10 14 eV= 100 TeV. As a rough estimate for the particle number Nat sea level in its dependence on the primary energy E 0 , onecan use the relationN = 10 −10 E 0 [eV] . (7.18)Only about 10% of the charged particles in an extensive airshower are muons. The number of muons reaches a plateaualready at an atmospheric depth of 200 g/cm 2 (see also Fig.7.9 and Fig. 7.10). Its number is hardly reduced to sea level,since the probability for catastrophic energy-loss processes,like bremsstrahlung, is low compared to electrons because ofthe large muon mass. Muons also lose only a small fractionof their energy by ionization. Because of the relativistic timedilation the decay of energetic muons (E µ > 3GeV)intheatmosphere is strongly suppressed.Figure 7.26 shows schematically the longitudinal de-velopment of the various components of an extensive airshower in the atmosphere for a primary energy of 10 15 eV.The lateral spread of an extensive air shower is essentiallycaused by the transferred transverse momenta in hadronicinteractions and by multiple scattering of low-energy showerparticles. The muon component is relatively flat compared tothe lateral distribution of electrons and hadrons. Figure 7.27shows the lateral particle profile for the various shower components.Neutrinos essentially follow the shape of the muoncomponent.Even though an extensive air shower initiated by primaryparticles with energies below 100 TeV does not reach sealevel, it can nevertheless be recorded via the Cherenkov lightemitted by the shower particles (see Sect. 6.3 on gamma-rayastronomy). At higher energies one has the choice of variousdetection techniques.The classical technique for the measurement of extensiveair showers is the sampling of shower particles at sealevel with typically 1 m 2 large scintillators or water Cherenkovcounters. This technique is sketched in Fig. 7.28.longitudinal profileof a showerlateral distributionFig. 7.27Average lateral distribution of theshower components for N = 10 5corresponding to E ≈ 10 15 eVAuger projectIn the Auger project in Argentina 3000 sampling detectorswill be used for the measurement of the sea-level componentof extensive air showers. However, the energy assignmentfor the primary particle using this technique is not very